A steady increase in the number of transistors that can be built on a chip surface has gain importance in microelectronics during the past decades. The industry data show that the number of bits/chip will be increased from 1 kilobit (Kb) in the late 1960's to 1 Gigabit (Gb) by the end of the decade. It is recognized that device geometries both in the horizontal and in the vertical directions, must shrink steadily to allow such an increase in density to occur. One other benefit achieved by the scaling down of devices to smaller geometries is the increase in circuit speed. The smaller the minimum feature size, i.e., the minimum line-width or line-to-line separation that can be printed on the surface of a wafer, the larger the number of circuits can be placed on the chip resulting in a higher circuit speed.
One of the most frequently used processing techniques in microelectronics for fabricating integrated circuits (ICs) and very large scale ICs (VLSI) is lithography. The term lithography is used to describe a process in which a pattern is reproduced in a layer of material that is sensitive to photons, electrons or ions. The principle is similar to that used in photography in which an object is imaged on a photo-sensitive emulsion film. After development, the exposed regions of the film are left as a layer of metallic silver, while the unexposed regions are removed, resulting in a printed image of the object. While the final product is a printed image in photography, the image in microelectronics is typically an intermediate pattern which defines regions where material is to be etched or implanted.
The manufacturing process for IC devices is dependent upon the accurate reproduction of computer aided design (CAD) generated patterns onto the surface of a substrate. The replication process is typically performed by a lithographic technique, specifically an optical lithographic technique, that is preceded and followed by a variety of etching and ion implantation processes. Lithography is a critical step in semiconductor manufacturing because it is used repeatedly in a process sequence that depends on the device design. The lithographic process determines the device dimensions, which affect not only the quality but also its production volume and manufacturing cost. Lithography transforms complex circuit diagrams into patterns which are defined on the wafer in a succession of exposure and processing steps to form a number of superimposed layers of insulators, conductors and semiconductor materials. For instance, typically between 5 and 30 lithographic steps and several hundred processing steps between exposures are required to fabricate a semiconductor IC package.
A typical photolithographic process can be carried out on a wafer surface by the operating steps of first oxidizing the silicon surface to obtain a thin layer of silicon dioxide, then coating a layer of material that is sensitive to radiations such as ultraviolet light, electron beams or X-ray beams, then imprint a latent image on the material by a lithographic patterning technique, then amplify the latent image by an appropriate development method, the reserve areas delimiting the design to be reproduced on the silicon dioxide layer, then stabilize the image by an appropriate fixing method or a lithography method, and then plasma etching the silicon dioxide surface through openings obtained in the dioxide layer to obtain areas variously doped in the silicon dioxide subsequently defining two-dimensional geometric shapes on the surface of the substrate for the circuit, such as the gate electrodes, contacts, vias and metal interconnects.
Optical lithography, as commonly used in the manufacture of integrated circuits (ICs) and very large scale ICs (VLSI), involves a series of steps for obtaining complex etched structures on a wafer. An optical lithographic patterning process involves the illumination of a metallic coated quartz plate known as a photomask which contains a magnified image of the computer generated pattern etched into the metallic layer. An illuminated image is reduced in size and patterned onto a photosensitive film deposited on the device substrate. Each time the photolithographic steps are repeated, the accuracy of the process determines the quality and the yield of the IC fabrication process. The performance enhancement of advanced VLSI circuitry is increasingly limited by the lack of pattern fidelity in a series of lithographic and reactive ion etching (RIE) processes conducted at extremely small dimensions (e.g., in the sub-half-micron level). In a photolithographic process, a pattern is transferred from a photomask to a photosensitive film (i.e., a photoresist layer) on a wafer. In the RIE process, the pattern in the photoresist is in turn transferred into a variety of conductive or insulating films on the wafer substrate. A successful fabrication process for integrated circuits is therefore closely dependent upon a successful lithographic technique.
During each lithographic step, deviations may be introduced to distort the image that the photomask transfers to the wafer surface. For example, as a result of an optical interference and other interferences which occur during a pattern transfer, images formed on a wafer surface deviate from the original dimension and shape of the photomask that was stored in the computer. The deviations depend on the characteristics of the pattern as well as a variety of processing parameters. Since deviations significantly affect the performance of a semiconductor device, different techniques have been developed to focus on methods of compensation for the optical proximity effect so that they can be included in a CAD file to improve the image transfer process.
Other processing parameters during a photolithography step may also affect the accuracy of the transfer of images. For instance, when the lithography process is carried out in a stepper apparatus for transferring an image from a photomask to a photoresist layer on the wafer, the absolute flatness of an exposure table on which the wafer is positioned is also a critical factor. It has been found that even with the presence of a few dust particles, a local defocus on the wafer positioned on the table may lead to serious die loss on the wafer due to poor imaging. The exposure table, i.e., commonly known as the E-table, is designed to minimize the dust particle problem. For instance, the surface of the exposure table is designed in a grooved pattern to minimize the chances of dust particles contacting a wafer positioned thereon. While dust particles that have fallen into the grooves should not present a problem to the focus of the stepper, the particles may be carried out of the grooves and fall on top of the table by any air disturbance in the stepper. The dust particles then cause local defocus when the wafer is exposed due to a change in focus length by the presence of the particles. Other factors, such as static electricity or electrostatic attraction may also cause the dust particles to move from the bottom of the grooves to the top surface of the exposure table.
In order to thoroughly clean the top surface of an exposure table in a stepper, a cleaning tool is frequently provided by the stepper manufacturer. A typical cleaning tool supplied is a small vacuum apparatus equipped with numerous vacuum openings on a contact surface. One of such cleaning tool is shown in FIGS. 1A and 1B.
FIG. 1A is a plane view of a contact surface of a conventional vacuum apparatus 10 indicating a contact surface 12 fabricated of a metallic or a ceramic material and provided with a centrally located aperture 14 as a vacuum passage. A cross-sectional view of the cleaning apparatus 10 is shown in FIG. 1B. A vacuum passageway 16 is used for establishing fluid communication with the vacuum aperture 14 such that a vacuum force can be provided at the aperture 14. The vacuum apparatus 10 is not always effective in removing fine particles from a grooved surface of a stepper exposure table. For instance, as shown in FIG. 2A, a dust particle 20 may be present at the shoulder portion of a grooved surface 22. The fine dust particle normally has a particle size of a few microns. In the grooved surface 22, the bottom surface 24 of the grooves are separated by sidewalls 26 of the grooves which also have a top surface 28. When a conventional vacuum apparatus 10 is pressed upon the grooved surface 22, i.e., with the contact surface 12 of the vacuum apparatus 10 intimately touching the top surface 28 of the grooved surface 22, the mechanical motion of the vacuum apparatus 10 first pushes the dust particle 20 on the shoulder portion such that it falls into the bottom surface 24 of the groove surface 22. This is shown in FIG. 2B. As the vacuum apparatus 10 is pushed further along the top surface 28, as shown in FIG. 2C, the vacuum aperture 14 is moved to a position that it is on top of the dust particle 20. However, with the dust particle 20 sitting on the bottom surface 24 in the grooves, the vacuum force exerted through the aperture 14 is insufficient to loosen the dust particle 20 from the bottom surface 24 and to suck it up into the vacuum aperture 14. As a result, after the vacuum apparatus 10 is further pushed along the top surface 28, the dust particle 20 remains on the bottom surface 24 of the grooved surface 22. This is shown in FIG. 2D. The ineffectiveness of the conventional apparatus 10 in removing fine dust particles is therefore fully demonstrated in FIGS. 2A.about.2D.
It is therefore an object of the present invention to provide a cleaning apparatus for removing fine particles from an uneven surface that does not have the drawbacks or shortcomings of the conventional cleaning apparatus.
It is another object of the present invention to provide a cleaning apparatus for removing fine particles from an uneven surface such that the apparatus can be advantageously used in removing particles having a size of several microns.
It is a further object of the present invention to provide a cleaning apparatus for removing fine particles from an uneven surface that can be advantageously used in semiconductor processing equipment such as in a photolithographic apparatus.
It is another further object of the present invention to provide a cleaning apparatus for removing fine particles from an uneven surface wherein the uneven surface is an exposure table having a grooved top surface used in a stepper.
It is still another object of the present invention to provide a cleaning apparatus for removing fine particles from an uneven surface wherein both air holes and vacuum holes are provided in a contact surface of the apparatus.
It is yet another object of the present invention to provide a cleaning apparatus for removing fine particles from an uneven surface wherein the apparatus is effective in picking up fine particles by vacuum apertures after the particles are first loosened from the uneven surface by an air flow provided by the cleaning apparatus.
It is still another further object of the present invention to provide a method for removing fine particles from an uneven surface which can be carried out by first loosening the particles from an uneven surface by an air flow and then picking up the particles into a vacuum aperture of the cleaning apparatus.
It is yet another further object of the present invention to provide a cleaning apparatus for removing fine particles from an uneven surface wherein the apparatus may be constructed by three sections stacked together for providing a plurality of vacuum openings and air holes in a contact surface of the cleaning apparatus.